1. Field of the Invention
This invention relates to a focal-plane shutter for cameras which has one or two shutter blade groups, each including a plurality of arms, one end of each of which is pivotally mounted to a shutter base plate, and at least one blade pivotally supported by the arms.
2. Description of Related Art
In some focal-plane shutters used in recent years, two blade chambers are provided between three plate members, called a shutter base plate, an intermediate plate, and an auxiliary base plate, respectively, and two shutter blade groups, called a first blade (group) and a second blade (group), are separately placed in these blade chambers. Such focal-plane shutters are used in digital still cameras and silver salt film cameras as well. The focal-plane shutters used in the former cameras are known as ones in which a single shutter blade group is placed in a blade chamber provided between the shutter base plate and the auxiliary base plate.
In either type of the focal-plane shutters mentioned above, the fundamental structure of the shutter blade group is the same, and it is common practice that one or more (usually, a plurality of) blades are pivotally supported by two arms (the structure of three arms is also proposed), one end of each of which is pivotally mounted to the shutter base plate, and thereby a link mechanism of parallelogram is constructed. In the type that the two shutter blade groups are provided, a blade pivotally mounted at the foremost arm end of each shutter blade group is constructed as a slit-forming blade. The blades of the shutter blade groups relative to their respective arms have the same pivotal support structure, in which holes provided in the arms and blades are made to overlap and the tops of joint shanks which are rivet parts are inserted into the holes from the arm side and are secured to the blades by caulking work.
As a result, the joint shanks integrated with the blades and the arms are in a mutually rotatable state, but individual blades are pivotally supported by the two arms and thus only when the arms are actuated, relative rotation takes place at pivotal support portions. The caulking work mentioned above is such that each of caulking portions does not project from the sliding surface (a surface different from a surface opposite to the arm) of the blade by changing the shape of the blade, but the head of the joint shank projects from the arm. The structure of such a pivotal support portion is well known and is disclosed, for example, in Japanese Patent Kokai No. Hei 7-333684, but the auxiliary base plate is here referred to as a cover plate.
In the case of the focal-plane shutter which has two shutter blade groups, the sliding surface of each blade is directed toward the intermediate plate in the corresponding blade chamber, and thus the arms for one shutter blade group are placed on the shutter base plate side of the blades, while the arms for the other shutter blade group are placed on the auxiliary base plate side of the blades. Specifically, one shutter blade group causes all the heads of the joint shanks to project from the arms toward the shutter base plate side, while the other shutter blade group causes all the heads of the joint shanks to project from the arms toward the auxiliary base plate side.
Hence, where the shutter of this type is mounted in a camera, either one of the shutter blade groups causes the heads of the joint shanks to project toward the base plate located on the photographer side (namely the image sensor side or the film side) even though either the shutter base plate or the auxiliary base plate is located on the object side. On the other hand, in the focal-plane shutter which has only the single shutter blade group, the heads of the joint shanks can be located on the object side with respect to the camera, but a case occur in which they must be located on the photographer side because of space for incorporating the shutter in the camera.
In the shutter base plate, the intermediate plate, and the auxiliary base plate which constitute the blade chamber, apertures for exposure are provided at about their middle portions so that an exposure aperture of light from the object is determined by one or more of the apertures for exposure. Either of the focal-plane shutters described above is generally constructed so that since it is necessary to downsize the entire shutter unit, at least, the pivotal support portion at the foremost arm end is moved from the outside into the inside of the aperture and is removed from the inside to the outside of the aperture when each shutter blade group is actuated.
In the silver salt film camera as well as in the digital still camera, if dirt or dust particles adhere to the imaging surface of an image sensor or the photosensitive surface of a film, a favorable object image cannot be obtained. However, most of dirt or dust particles, usually produced in the camera, are so fine that it is difficult to see with the eye. Most of dust particles penetrating from the exterior into the camera are also fine, but sometimes contains large ones. In a conventional silver salt film camera, even when relatively large dust particles penetrate into the camera to adhere to the photosensitive surface of the film, only the frame of the film is affected, and thus little attention has been devoted to the dust particles. However, since the defect of the digital still camera to be described later has been known, special attention has frequently been devoted to this defect in recent years.
On the other hand, in the digital still camera, even though dirt or dust particles are extremely fine, attention is arrested. Specifically, when dirt or dust particles are fine, attention is not initially arrested, but they gather gradually on, and adhere to, the imaging surface and a filter placed in front thereof. In this way, the influence of the dust particles on the entire photographing image is gradually increased. Consequently, in the digital still camera, it is necessary to make some provision for any fine dirt or dust whatever. Thus, in the fabrication of the focal-plane shutter, as well as in the camera body itself, it is required to make provision for the structure as far as possible, and many proposals of such provisions have been offered.
Here, consider fine wear dust produced by the actuation of the focal-plane shutter. In the focal-plane shutter, it is generally known that extremely fine wear dust is produced from the sliding and abutting portions of the shutter in the actuation. However, it is very difficult to design the shutter so that the wear dust is not completely produced. The wear dust produced from the abutting portions is larger and more than that produced from the sliding portions. As such, it is necessary that the wear dust chiefly produced from the abutting portions is made to vanish completely or is reduced to a minimum. In this case also, it is advantageous that provision is first made with respect to a place where the wear dust is easy to reach the image sensor or the film. Therefore, it is most important to make provision in the proximity of the aperture of the base plate located on the photographer side, of the shutter base plate and the auxiliary base plate.
When the structure of the focal-plane shutter is seen from such a viewpoint, the focal-plane shutter is constructed so that the heads of the joint shanks used to pivotally support the blades with respect to the arms project from the arms toward the base plate on the photographer side in a state where the shutter is incorporated in the camera. Consequently, the most important problem is that the heads of the joint shanks moved inside the aperture during the actuation of the shutter blade group abut against the edge of the aperture of the base plate on the photographer side (the reason why this abutment is caused is well known and is also set forth in the prior art publication mentioned above) to produce the wear dust. Thus, it is conceivable that, as disclosed in Japanese Patent Kokai No. Hei 9-160091, convex portions are provided on the arm side or the base plate side so that the heads of the joint shanks do not abut against the edge of the aperture.
The arms are used in such a way that, in order to withstand a severe shock caused at the stop of an exposure operation and to achieve lightweight and high-speed design, carbon tool steel (SK4) stipulated in JIS (Japanese Industrial Standard) is heat-treated and after it is immersed in an alkaline solution to form a black oxide film, chromate treatment is applied to the steel, or after pure titanium of the second kind stipulated in JIS is nitride-treated, black painting is applied to this metal. Consequently, when the convex portions are provided on the arm side as in Kokai No, Hei 9-160091, the problem arises that the weight of the shutter blade group is increased, which becomes disadvantageous for achieving the high-speed design of the exposure operation.
As the base plate (the shutter base plate or the auxiliary base plate), to achieve the lightweight design of the entire shutter unit, aluminum or synthetic resin material softer than the arm has come into prominent use in recent years. Hence, the problem is caused that when the convex portions are provided to the base plate by punching work or simultaneous molding work, the arm made of a relatively hard, thin plate material abuts against, and slides along, the convex portions and thereby the surfaces of the convex portions are ground to produce the wear dust.
It is, therefore, an object of the present invention to provide a focal-plane shutter for cameras in which even when the heads of the joint shanks for pivotally supporting blades with respect to the arms are constructed to project from the arms toward the base plate on the photographer side in a state where the shutter is incorporated in the camera, the heads of the joint shanks moved inside the aperture during the actuation of the shutter blade group do not abut against the edge of the aperture, and even when the arms abut against, and slide along, the convex portion provided on the base plate, the wear dust is not produced.
In order to accomplish the above object, the focal-plane shutter for digital still cameras of the present invention includes two base plates provided with a blade chamber between them, each having an aperture for exposure at about the center, and a shutter blade group having a plurality of arms pivotally mounted to one of the two base plates and at least one blade pivotally supported by means of a plurality of joint shanks with respect to the plurality of arms, placed in the blade chamber so that the heads of the joint shanks project toward the photographer side. In this case, of the two base plates, the base plate placed on the photographer side has at least one convex portion metal-plated on its the object-side surface, and the arms slide along the convex portion and are shifted to the object side immediately before the heads of the joint shanks moved inside the aperture in the actuation of the shutter blade group reach a position corresponding to the edge of the aperture.
In this case, it is desirable that the surface hardness of metal plating of the convex portion is almost the same as that of each arm. In particular, the substance of the metal plating of the convex portion is palladium, and when the arm is made in such a way that after carbon tool steel is immersed in an alkaline solution to form a black oxide film, chromate treatment is applied to the steel, or titanium is nitride-treated, the optimum arm can be obtained.
In the present invention, the convex portion may be constructed as a member configured so that the object-side surface is spherical with respect to a separate member mounted to the base plate on the image sensor side. Alternatively, the base plate on the image sensor side is made of synthetic resin, and the convex portion may be constructed as a member integrally configured so that the object-side surface is spherical.
Further, the present invention may be designed so that the blade chamber provided between the two base plates is partitioned by an intermediate plate, the shutter blade group is placed between the intermediate plate and the base plate on the photographer side, and another shutter blade group pivotally supporting at least one blade by means of joint shanks with respect to a plurality of arms pivotally mounted to the one base plate is placed between the intermediate plate and the base plate on the object side.
According to the present invention, in the focal-plane shutter for cameras which is provided with at least one shutter blade group pivotally supporting at least one blade by means of the joint shanks with respect to the plurality of arms, the convex portion metal-plated is provided on the blade-chamber-side surface of the base plate placed on the photographer side, and before the joint shank for pivotally supporting the blade at top of the arm is moved inside the aperture of the base plate and reaches the edge of the aperture, the arm strikes on the convex portion so that its operation path is shifted to the object side. Thus, it is avoidable that the arm abuts against, and slides along, the convex portion to thereby produce the wear dust. Moreover, although the heads of the joint shanks for pivotally supporting the blades project from the arms toward the image sensor side, they do not abut against the edge of the aperture and hence the wear dust ceases to be produced as in a conventional shutter. Consequently, it is avoidable that the wear dust is produced immediately before the image sensor or the film and adheres thereto.
This and other objects as well as the features and advantages of the present invention will become apparent from the following detailed description when taken in conjunction with the accompanying drawings.